The "hash" (or "hash value") is a fixed-length value obtained by converting another value. Hashes can only be converted in one direction, so it is impossible to convert hashed data back into its source data. Changing the source data by even one digit completely changes the hash, so it is impossible to guess the source data based on its hash.
The hashes used for Bitcoin are SHA-256 and RIPEMD-160. These hashes are meant to be calculated by GPU. However, there are many altcoins (alternatives to Bitcoin) that were developed later, where Scrypt-type hashing is used, which makes mining with a regular PC easier.
In general, passwords are stored in a hashed state. By doing this, operators cannot know users' passwords and they remain secure even if the hashed passwords are leaked, so this is a crucial part of the design.